This invention relates to an idling speed control system for internal combustion engines, and more particularly to a system of this kind which is capable of stabilizing the rotational speed of the engine at idle by controlling the quantity of intake air supplied to the engine.
An internal combustion engine for automotive vehicles is so constructed that the output power and rotational speed thereof are controlled by controlling the quantity of intake air by the use of a throttle valve. In an engine having a carburetor, a throttle valve is generally mounted in the carburetor and so arranged that the opening thereof can be adjusted by an idling opening adjusting bolt screwed in the body of the carburetor. The idling opening of the throttle valve is adjusted, by the use of the idling opening adjusting bolt, to a suitable value at the time of manufacture or maintenance operation of the engine, and therefore the idling opening thus set by the bolt cannot be arbitrarily further adjusted by a driver during operation of the engine.
Since the idling opening of the throttle valve thus has an adjusted fixed value, the rotational speed of the engine is kept constant, if the load on the engine does not vary during idling operation of the engine. However, if the load on the engine varies due to variations in the load on the generator for charging the battery or in the load on the automatic transmission, or due to switching-on and -off of the compressor in the air-conditioner, the rotational speed of the engine correspondingly varies, which makes it difficult to obtain stable idling speed and sometimes results in engine stalling. It is therefore necessary to set a desired idling speed at such a high value as to keep the engine always operating in a stable idling condition, without being influenced by the variations in the engine load. However, if the desired idling speed is set at such a high value, there can occur problems such as occurrence of large noise during idling operation of the engine, and increase of the fuel consumption.
Further, as shown in FIG. 6, at so-called snap deceleration, e.g. if the accelerating pedal is stepped on to abruptly increase the engine rotational speed to 1500 rpm when the engine is operating at an idling speed, e.g. 750 rpm and then the accelerator pedal is suddenly released from its stepped-on state, the engine rotational speed can suddenly drop below the desired idling speed, which renders the engine operation unstable and sometimes causes engine stalling.
To solve such problems, it has conventionally been proposed e.g. by Japanese Provisional patent Publication No. 58-155255 to control the throttle valve opening during idling operation of the engine by the use of a pulse motor. Another method of controlling the idling speed of the engine has been proposed by Japanese Provisional patent Publication No. 59-155547, which comprises detecting the rotational speed of the engine by the use of a predetermined crank angle signal, calculating the difference between the detected engine rotational speed and a desired idling speed, and controlling the quantity of intake air bypassing the throttle valve by controlling the duty ratio of a control valve with a control signal corresponding to the difference thus calculated, so as to attain the desired idling speed.
The above proposed methods, however, require complicated control systems as well as expensive control devices and control valves, and thereby are not practical.